Diode pumped fiber lasers have a long, thin geometry that allows better heat removal than the geometry of bulk solid state lasers. Currently, pump light, often piped through fibers from pump lasers, enters an outer core of the fiber laser, where it is confined and redirected to pass through an inner core of the fiber laser where it excites laser active material to produce and amplify light. Pump light may enter the fiber either through the end of the fiber or through the side of the fiber.
The output wavelength of many solid state lasers is between about 1-2 μm. Semiconductor materials can be doped with dopants such as Nd, Er, Yb, Vn to achieve a laser output within this wavelength range. Therefore, the following text assumes that the below-described fiber lasers have an output wavelength in this range. In case the output wavelength differs from this assumption, dimensions of the fiber laser are scaled appropriately with the output wavelength.
To convert pump light power into output laser power at the desired wavelength over the length of the fiber, a “double-clad fiber laser” has been used. Such a double-clad fiber laser typically consists of a single-mode core (for the output laser wavelength) that is embedded in a multi-mode cladding (for the pump laser wavelength), which itself can be embedded in an outer cladding.
The multi-mode cladding of a fiber laser has a diameter that is on the order of several ten to several hundred μm in diameter. The multi-mode cladding transmits the light from pump laser diodes that are either coupled in along the side of the fiber (i.e., an “side-pumped fiber laser”) or are located at one or both ends of the fiber (i.e., an “end-pumped fiber laser”).
The single-mode core is on the order of several Am in diameter and carries the lasing dopant. The dopant absorbs the pump wavelength and creates gain for the output laser wavelength inside the core. Because the core can only carry the lowest order waveguide mode with low losses, lasing in higher order modes does not occur, and diffraction-limited beam quality can be achieved from a single-mode fiber laser. The inner core, active region of such a single-mode fiber laser typically has a diameter that is chosen so that the lowest order Gaussian mode is the only mode that can propagate in the active core without substantial losses. In other words the diameter is chosen so that the cut-off frequency for any higher order mode but the lowest order Gaussian mode is above the lasing frequency of the active medium. Therefore these modes cannot propagate confined to the active core.
In general, for such a double clad fiber laser to work well, the pump wavelength should efficiently penetrate both the cladding and the core, while the output laser wavelength should be carried only in the core. The difference in the index of refraction between the core and the cladding layer ensures that the light of the output laser wavelength is confined to the core region.
For many material processing applications (e.g., cutting and welding of metals), high continuous wave (“cw”) power (multi-kW) and high beam quality (near the diffraction limit) are desirable. The fiber geometry is well suited for multi-kW operation, because excessive heat can be efficiently removed over the length of the fiber. However, the radiation intensity Imin (measured in Watts per square centimeter) within the fiber is proportional to the output power PL (measured in Watts) for a given laser wavelength,Imin∝PL,and at very high intensities non-linear effects occur that effectively prohibit efficient laser operation. Because the diameter of the core of a typical single-mode fiber laser is limited by the wavelength of the output laser light, these conditions impose an effective upper power limit for single-mode, cw-operation, which currently is about 200 W.